It is well known in the art that the introduction of photographically useful compounds, such as photographic developers, couplers, development inhibitors, electron transfer agents, base precursors, fixing agents, i.e., ligand capable of binding silver, silver stabilizing agents and the like, into photographic elements can lead to premature reaction of the photographically useful compound with the other components of the photographic element. For example, placing conventional color developers, such as p-phenylenediamines and p-aminophenols, into sensitized photographic elements leads to desensitization of the silver halide emulsion and unsuitable fog. Much effort has therefore been directed at trying to produce effective blocked developers, which can be introduced in silver halide emulsion elements without deleterious desensitization or fog effects and which unblock chemically under conditions of development so that developer is free to participate in color forming (dye forming) reactions.
U.S. Pat. No. 3,342,599, to Reeves, discloses the use of Schiff base developer precursors. U.S. Pat. No. 4,157,915, to Hamaoka et al., and U.S. Pat. No. 4,060,418, to Waxman and Mourning, describe the preparation and use of carbamate blocked p-phenylenediamines. Color developing agents having .alpha.-ketoacyl blocking groups are described in U.S. Pat. No 5,019,492.
All of these approaches and inventions have failed in practical product applications because of one or more of the following problems: desensitization of sensitized silver halide; unacceptably slow unblocking kinetics; instability of blocked developer yielding increased fog and/or decreased Dmax after storage; and the requirement of a dinucleophile, such as hydroxylamine, to initiate developer release.
The addition of a blocking group to a color developing agent results in an increase in molecular weight and generally results in a decrease in water solubility of the resulting blocked version of the parent color developing agent. As a result, the incorporation of these blocked developers in photographic elements is carried out using colloidal gelatin dispersions of the blocked developers. These dispersions are prepared using means well known in the art, wherein the developer precursor is dissolved in a high vapor pressure organic solvent (for example, ethyl acetate), along with, in some cases, a low vapor pressure organic solvent (such as dibutylphthalate), and then emulsified with an aqueous surfactant and gelatin solution. After emulsification, usually done with a colloid mill, the high vapor pressure organic solvent is removed by evaporation or by washing, as is well known in the art.